Digestion and Absorption

Question 1

what is the role of the intestinal epithelium barrier

Answer 1

1. Enable the absorption of nutrients

2. Control the passage of pathogens or toxins

Question 2

how is the internal epithelial barrier regulated

Answer 2

• Outer microenvironment by the microflora and chyme

- Inner microenvironment by immune cells, fibroblasts or the enteric nervous system and extrinsic nerve fibres)

Question 3

what is the functional structure of the enterocytes built by

Answer 3

specialised cytoskeletal proteins

Question 4

how much absorption of nutrients occurs In the small intestine

Answer 4

• 95% of absorption of nutrients occurs in the small intestine

Question 5

what is the rate of fluid movement in the digestive system

Answer 5

• ingested water (2 liters)
• liver and pancreatic secretions ( 2 liters)
• salivary gland secretions (1.5 liters)
• secretions by glands of the stomach and small intestines (3.5 liters).
• Small intestine absorbs 8.5L
• Colon 400ml
• Faeces 100ml

Question 6

describe what the role of the stem cells at the bottom of the crypt is

Answer 6

At the base of crypt you have stem cells, replicate all the time, gut is a harsh environment and many of the cells at the top of the crypt is removed therefore the stem cells produced replace them

Question 7

what are the 3 processes of digestion and absorption

Answer 7

1. luminal
2. mucosal phase
3. post absorptive phase

Question 8

describe the luminal phase

Answer 8

The ingested food is broken down by acid in the stomach, alkali in the small intestine and substrate-specific enzymes secreted by the gastric and small bowel mucosa and the pancreas

Question 9

describe the mucosal phase

Answer 9

is when pre-digested nutrients are selectively taken up at the brush border membrane of the enterocytes and then enter the intestinal cells.

Question 10

describe the post absorptive phase

Answer 10

transport of absorbed nutrients via lympathetics and the portal circulation to the rest of the body

Question 11

why is water needed in the digestive system

Answer 11

• Hydrolysis reactions of digestion
• Facilitation of absorption (brings products of digestion into close proximity to microvilli)
• Facilitation of propulsion of gut contents
• Combination with mucin granules to make mucus

Question 12

how do crypt cells assure that water will be in the gut (very important)

Answer 12

• Sodium and potassium and chlorine enter by cotransport
• Chloride enters lumen through CFTR channel
• Sodium is reabsorbed
• Negative CL- in lumen attracts sodium by paracellular pathway
• Water follows
• This is the system by which the crypt cells assure that water will be in the gut

Question 13

what nutrients are absorbed in the small intestine

Answer 13

• Carbohydrate and complex carbohydrates
• Lipids
• Proteins
• Vitamins

Question 14

how is carbohydrates digested and absorbed

Answer 14

1. Luminal phase
   – Split into shorter molecules by salivary (trivial – this is because as soon as you swallow it is inactivated ) and pancreatic (important) enzymes (e.g. maltase, amylase). The amlayse in the salvia does not work at low pH
   = disaccharides + limit dextrins
2. Mucosal phase
   - brush border enzymes complete digestion.
   - Glucose and galactose enter epithelial cells via sodium-linked secondary active transport across the apical membrane. Fructose enters by facilitated diffusion.
3. Post absorptive phase – the sugar exits the cells across the basolateral membrane by facilitated diffusion to the portal vein
   Potassium enters and sodium leaves

Question 15

what are the brush border enzymes

Answer 15

Sucrase

• lactase
• maltase
• limit dextrinase
• glucoamylase

Question 16

describe the digestion and absorption of lipids

Answer 16

1. Luminal phase
   - Lipid digestion begins in the mouth with lingual lipases and continues in the stomach where gastric lipase is added to the mixture.
   - With the help of Bile salts, pancreatic lipase digests triglycerides into free fatty acids, and monoglycerides.
2. Mucosal phase
   - Fatty acids and monoglycerides enter the enterocytes by simple diffusion as the membrane is lipid too
   - Inside the enterocytes the molecules are reassembled into triglycerides and are packaged into large particles called chylomicrons.
3. Post absorptive
   - The chylomicrons are secreted across the basolateral membrane by exocytosis. - The chylomicrons enter lymphatic capillaries. The flow of lymphatic fluid then carries the chylomicrons to the bloodstream.

Question 17

what do micelles contain

Answer 17

fat soluble vitamins and cholesterol

Question 18

what do bile salts do

Answer 18

• Bile salts enhance the digestive action of liases by breaking down fat globules into smaller droplets
• emulsification,

Question 19

how are bile salts recycled

Answer 19

• they are absorbed in the ileum and recycled by the liver

Question 20

what are communal bacteria

Answer 20

• they form the microbiome
• regulates of digestion
• participate in the synthesis of bile salts

Question 21

biles salts derived from…

Answer 21

commensal bacteria are directly linked to both diet and digestion and can be considered to be diet-dependent microbial products

Question 22

what is more efficient in emulsifying fats and why

Answer 22

Conjugated bile acids are more efficient in emulsifying fats because at intestinal pH they become more ionized that the unconjugated bile acids

Question 23

what do microbial enzymes do

Answer 23

deconjugate bile acids

Question 24

describe the digestion and absorption of proteins

Answer 24

1. Luminal phase
   Begins in the stomach with the action of pepsin. Pepsin is converted from pepsinogen by other pepsin molecules that are activated by HCl secreted by parietal cells.
   Protein digestion is completed in the small intestines by enzymes secreted by the pancreas.
2. Mucosal phase
   - Trypsinogen is converted into trypsin by enterokinase, a brush border enzyme.
   - Trypsin can then activate:
   - chymotrypsinogen -> chymotrypsin
   - procarboxypeptidase -> carboxypeptidase
   - need to have these proenzymes so it acts as a control mechanism to prevent digesting itself and the gut
   - amino acids enter the epithelia cells via sodium linked secondary active transport across the apical membrane
3. post absorptive phase
   - amino acids are then transporterd across the basolateral membrane by facilitated diffusion

Question 25

what are the two types of vitamins

Answer 25

fat soluble | water soluble

Question 26

describe how fat soluble vitamins are absorbed

Answer 26

- absorbed with lipids as they readily dissolve in lipid droplets, micelles and chylomicrons - INTO LYMPH FLUID

Question 27

describe how water soluble vitamins are absorbed

Answer 27

- most water soluble vitamins B,C follow the flux of water from the gut lumen through the mucosa

Question 28

what type of vitamin is B12

Answer 28

water soluble

Question 29

describe how B12 is absorbed in the terminal ileum

Answer 29

- binds to haptocorrin which is produced in the salivary glands - - Must be complexed with stomach-derived intrinsic factor (IF). - Transported into the portal circulation. - The vitamin is then transferred to transcobalamin II (TC-II/B12), which serves as the plasma transporter - Absorbed in the terminal ileum. - 60-80% IS INGESTED IN FEACES

Question 30

what circulations do the fat soluble and water soluble vitamins go in

Answer 30

fat soluble - lymph fluid | water soluble - portal vein

Question 31

describe how vibrio cholera causes secretory diarrhoea

Answer 31

- Gram negative comma shaped bacterium - Alpha part of the chloera toxin binds to adenylate cyclase - Activates adenylase cyclase - Makes cAMP - This activates the CFTR receptor and causes chlorine to go into the lumen - Therefore, sodium is excreted to maintain electroneutrality through the paracellular pathway therefore water follow - Consistent pour of water into the lumen this is called secretory diarrhoea

Question 32

how do you treat cholera

Answer 32

Oral rehydration - Drink that contains sodium, glucose and amino acids - The sodium glucose and sodium amino acid receptors are not effected by cAMP - The pump pumps the sodium out as you don’t want sodium in the cell and it is pumped out into the extracellular fluid - Therefore water follows into the extracellular fluid

Question 33

what causes generalised malabsorption

Answer 33

- commonly results from small intestine | - can also result from pancreatic disease or conditions in other organs such as liver disease

Question 34

what does small intestinal mucosal disease cause

Answer 34

reduced absorptive area

Question 35

what are examples of small intestinal mucosal disease

Answer 35

- coeliac disease - post infections malabsorption - crohns disease

Question 36

what is the most common cause of malabsorption in the economically developed world

Answer 36

coeliac disease

Question 37

what causes malabsorption in the developing world

Answer 37

- actue or chornic infection - post infectious malabsorption and tropical sprue - may be initiated by a viral infection - may be associated with chronic bacterial infection of the upper gastrointestinal tract.

Question 38

what is specific malabsorption

Answer 38

specific malabsorption is the failure of the processes governing absorption of one class or type of nutrient.

Question 39

what is an example of specific malabsorption

Answer 39

- For example genetic or acquired failure to absorb dissachride sugars or vitamin B12

Question 40

disaccharide deficiencies are ...

Answer 40

generally genetic and most of the world’s population is lactase deficient

Question 41

describe how disaccharide deficiency is caused

Answer 41

- Mutation in the gene LCT: Affecting the ‘mucosal phase’ of dissacharides absorption (lactase is in the brush border). - Glucose cannot be converted to galactose - Mutation in the gene LCT: - Mutation in the gene SLC5A1: - SLC5A1 encodes a member of the sodium-dependent glucose transporter SGLT1 (Sodium dependent GLucose Transporter one). Mutations in SLC5A1 cause Glucose-galactose malabsorption.

Question 42

what is the consequences of disaccharide deficiency

Answer 42

- Unabsorbed disaccharides reaching the colon adds to the osmotic load and causes watery diarrhoea. - Fermentation of sugars in the intestine causes gaseous distension and adds to the watery diarrhoea. Produces osmotic diarrhea